Frequency mixers may be included in many types of electronic systems. For example, frequency mixers in radio systems downconvert a received radio frequency (RF) signal by combining the RF signal with a local oscillator (LO) signal. The combination of the RF signal and the LO signal yields an intermediate frequency (IF) signal, which has a frequency corresponding to a difference between the RF and LO signals. Also, a frequency mixer may be employed to combine a modulated IF signal with an LO signal to upconvert the IF signal to a desired RF frequency for transmission. In some systems, a mixer may be employed for both upconversion of a transmit signal and downconversion of a receive signal.
Some important parameters of a mixer's performance include conversion loss, compression point, third-order intercept point (IP3), and port-to-port isolation. The values of these parameters normally depend on a variety of factors, including the mixer topology and the performance of the components employed in the mixer (e.g., diodes, field effect transistors (FETs), etc.). For example, in general a diode mixer exhibits a greater conversion loss and a moderate IP3 compared to a FET mixer, but generally has better LO-RF port isolation (lower LO leakage to the RF port) than an FET mixer over a broad frequency band.
If there is a need for lower LO leakage at the RF port, in general a balanced mixer topology is employed.
FIG. 1 is a block diagram of a mixer 100, which includes LO-port 110 for receiving an LO signal, RF-port 120 for inputting a received RF signal (and/or for outputting an RF signal for transmission) and IF-port 130 for outputting a downconverted IF signal (and/or for receiving an IF signal to be upconverted for transmission). The mixer 100 also includes a balun 105, a mixing circuit 115 comprising first and second mixing devices 125 and 135, and a diplexer 145.
Balun 105 is provided to improve LO-port isolation, and to reduce LO energy leakage to RF port 120. Balun 105 has an unbalanced LO-port 110 receiving an unbalanced LO signal, and provides a balanced LO output, comprising an in-phase LO signal on in-phase LO output 105a and an out-of-phase LO signal on out-of-phase LO output 105b. In a beneficial arrangement, the in-phase LO signal and the out-of-phase LO signal have substantially the same amplitude as each other, and the out-of-phase LO signal is phase shifted by about 180 degrees with respect to the in-phase LO signal.
First and second mixing devices 125/135 each have a first port 125a/135a and a second port 125b/135b. Input ports 125a/135a are connected respectively to in-phase and out-of-phase LO outputs 105a and 105b. 
Diplexer 145 has RF-port 120, IF-port 130, and a common port 145a. Common port 145a of diplexer 145 is connected to second ports 125b/135b of first and second mixing devices 125/135 via common node “C.”
Diplexer 145 performs frequency separation to enable RF and IF signals to be received and sent on different frequencies.
In mixer 100, balun 105 is employed to cancel out LO leakage thru mixing circuit 115 at common node C. Because the LO signal is fed to mixing circuit 115 through balun 105, which ideally provides in-phase and out-of-phase LO signals that are 180 degrees out of phase with respect to each other, any LO signals that leaks through first and second mixing devices 125/135 cancel each other out at common node “C.”
However, in practice, it is not possible to produce an ideal balun, and a real balun has some amount of amplitude and phase imbalance between the in-phase LO signal and the out-of-phase LO signal. As a result of this imbalance, the LO leakages through first and second mixing devices 125/135 do not cancel each other out exactly at common node “C.” As a result, some LO energy leaks onto RF port 120. The amount of LO leakage at RF port 120 depends on the amount of phase and amplitude imbalance in balun 105.
FIGS. 2A-C are graphs illustrating simulated local oscillator (LO) isolation for the mixer of FIG. 1 as a function of frequency for different levels of phase imbalance in balun 105. FIG. 2A illustrates a case where the phase imbalance is 0 degrees (ideal balance). In that case, it can be seen that the LO-to-RF port (L-R) isolation is extremely high—between 230-300 dB over a frequency range from about 4-20 GHz. FIG. 2B illustrates a case where the phase imbalance is only 1 degree. In that case, it can be seen that the L-R isolation has been reduced so as to be between 49-56 dB over a frequency range from about 4-20 GHz. FIG. 2C illustrates a case where the phase imbalance is 5 degrees. In that case, it can be seen that the L-R isolation has been reduced so as to be between 33-40 dB over a frequency range from about 4-20 GHz.
From FIGS. 2A-C, it can be seen that L-R isolation is very sensitive to balun performance.
With mixer 100, it is possible to achieve L-R isolation of 45-50 dB or more over a narrow frequency bandwidth by fine tuning the balun to have a phase imbalance of about 1% over the narrow frequency band. However, over a wide frequency bandwidth, a phase imbalance of 5% is considered to be very good, and this typically only yields L-R isolation numbers in the range of 30-40 dB. Nevertheless, for many applications, an L-R isolation of >25 dB is considered acceptable, and so the mixer 100 is able to meet these requirements. Furthermore, some manufacturers employ a bandpass filter after the mixer if greater LO rejection is required.
However, with the growing demand for new applications and lower cost devices, there is an increasing desire to minimize the number of filters employed, and there is a demand for broadband mixers with greater L-R isolation.
To improve L-R isolation, a mixer may apply a slight DC offset at the IF to cancel out balun imperfections.
FIG. 3 illustrates a test set-up for a mixer 300 employing a DC offset. Mixer 300 employs bias-tees 302 and 304 to add first and second DC offset voltages to first and second IF signals from first and second IF signal generators 332 and 334. LO signal generator 312 provides an LO signal to LO-port 310, and an RF output at RF-port 320 is coupled to the input 372 of a spectrum analyzer 370.
FIG. 4 illustrates improvement in LO isolation that can be provided by the mixer of FIG. 3 when appropriate DC voltages are applied to the bias-tees 302 and 304. The left side of FIG. 4 shows the LO and RF levels at spectrum analyzer 370 when no DC offset voltages are applied to bias-tees 302 and 304. The right side of FIG. 4 shows the LO and RF levels at spectrum analyzer 370 when selected DC offset voltages are applied to bias-tees 302 and 304. As can be seen in FIG. 4, the LO level at RF port 320 can be substantially improved by applying appropriate voltages to bias-tees 302 and 304.
Although L-R isolation may be improved in theory by the arrangement of FIG. 3, the arrangement is quite sensitive and requires continuous adjustment to the DC offset voltages when there are any changes to any environmental conditions, such as due to temperature, aging of components, etc. Accordingly, this arrangement is not very effective for a wide range of operating conditions.
So it would be desirable to provide an arrangement which can reduce LO leakage to an RF port of a mixer. It would further be desirable to provide such an arrangement which can operate over a wide frequency bandwidth and a wide range of operating conditions.